Ion implantation is a standard technique for introducing conductivity—altering impurities into semiconductor wafers. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity. The ion beam may be distributed over the wafer area by beam scanning, by wafer movement, or by a combination of beam scanning and wafer movement. Introducing the impurities at a uniform depth and density into the wafers is important to ensure that the semiconductor device being formed operates within specification.
One factor that can affect the uniformity of the impurity dose into the wafer is the ion beam current. An unexpected fluctuation in ion beam current may degrade the resulting uniformity of the impurity dose. Accordingly, various conventional ion beam current monitoring systems have been developed. One conventional ion beam current monitoring system utilizes a Faraday sensor disposed proximate the wafer that periodically samples the ion beam current as the ion beam is moved relative to the wafer. A drawback with this approach is that when the ion beam is actually striking the wafer, the beam current is not being monitored. Therefore, if an unexpected fluctuation in beam current occurs during this time and then corrects itself by the time the ion beam strikes the sampling Faraday sensor, the magnitude and duration of the unexpected fluctuation would not be known.
Another conventional ion beam current monitoring system may continuously monitor the magnetic field of the ion beam. Fluctuations in the magnetic field may then be correlated to changes in ion beam current. A drawback of this approach is its limited applicability to relatively high beam current values since a relatively high beam current is necessary to provide a sufficient magnetic field to be sensed.
Yet another conventional ion beam current monitoring system may continuously monitor power supply currents via a total return current in the flight tube of a batch tool. The measured total return current in the flight tube may then be used to provide an indication of the total ion beam current leaving the fight tube. A drawback with this approach is the precision of the correlation between the measured total return current to the actual ion beam current. The actual ion beam current delivered to the wafer in the end station may be affected by other indicia not detected by the total ion beam current leaving the flight tube.
Accordingly, there is a need for new and improved methods and apparatus for ion beam current monitoring.